From SBAMUG WIKI
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This should be done:
- Before and after a major software upgrade. Like adding MS Office or upgrading Mac OS X to the next version.
- Before cloning or a major Backup
- List from Apple of permissions you can ignore: 
This is one of those things you should do if you don't remember when you did it last time. It should also be looked at anytime you change a eMail address in your Address book or you fill-in addresses show issues.